1. Field of the Invention
The present invention generally relates to toner concentration control technique for use in electrophotographic equipment.
2. Description of the Related Art
There has been proposed a toner concentration detection technique making use of electric conductivity of liquid developer in Japanese Patent Unexamined Publication No. 3-295453. The electric conductivity is measured using alternating current because direct current measurement causes movement of ionic carriers and polarization by which voltage drops are caused around electrodes. The measurement frequency is determined depending on the frequency response of the object. In the case of liquid developer, a frequency of 1 kHz may be preferably used.
However, there occurs an increase in the number of ionic contaminants or the like due to deterioration of liquid developer. Such Ionic contaminants or the like become a factor that substantially influences the measurement, resulting in a lower degree of measurement accuracy.